This invention pertains generally to the formation of images by scanning, as in a laser engraving system, and more particularly to a system and method for eliminating moire interference in the formation of halftone images.
In recent years, laser engraving systems have been employed in the production of plates for the printing of newspapers and the like. In such systems, all or any part of the copy to be printed can be input by scanning from a copyboard, or it can be input through a computer or another suitable source. The information which is input is either stored or used immediately to control the modulation of a laser beam which scans the output medium to form the desired image.
When a halftone image is scanned, the resulting video signal has a frequency spectrum which is concentrated near one central frequency. If the video signal is sampled at a constant rate, one or more beat frequencies are generated between the video signal and the sampling frequency. These beat frequencies can produce interference in the form of wavy patterns, commonly known as moire patterns, which are visible to the eye in the output image.
There have been attempts to eliminate the moire interference from halftone images by increasing the sampling frequency so that the magnitude of the disturbances produced by the beat frequencies will be small and therefore not visible in the output image. While this technique may work satisfactorily in some instances, it is not always desirable to increase the sampling frequency, since a higher sampling frequency may either require a longer time to transmit an image or result in a higher signal-to-noise ratio in transmission in the sampling frequency and with use of same as the transmission frequency bandwidth.